1. Field of Invention
This invention relates generally to a multi-channel transparent jacket for accommodating strips of microfilm, the jacket being constituted by front and rear panels of flexible plastic film material joined together by a parallel array of ribs which define the channels, and more particularly to a jacket of this type in which the panels are formed of polyethylene terephthalate film having a friction-reducing surface layer thereon imparting non-blocking characteristics to the jacket, whereby when such jackets are stacked they do not stick to each other, and it is possible, therefore, to extract one jacket from the stack without an adjacent jacket being carried along by this action.
2. Status of the Art
The Engelstein U.S. Pat. No. 3,238,655 entitled "Microfiche Master" discloses a transparent jacket formed by two plastic panels laminated together by a parallel array of ribs which define a series of chambers or channels adapted to accommodate micro film strips. The loaded, multi-channel jacket functions as a microfiche master from which reference copies may be made. This is effected by contact-printing through the front panel which is quite thin and has good optical properties, the back panel being thicker to impart body to the jacket. Such microfiche masters are highly useful in storing and disseminating information.
The Dorman U.S. Pat. No. 4,452,666 shows a similar multi-channel transparent jacket, but in this instance, the ribs are formed by extruding molten beads of plastic material and directing these beads between the panels which are then compressed to produce in situ ribs integral with the panels. The present invention is applicable to any transparent jacket formed by front and rear panels of polyethylene terephthalate film material. It is also applicable to file cards and other record sheets made of polyethylene terephthalate film or similar polymeric material.
The Engelstein-Dorman U.S. Pat. No. 3,713,535 discloses a random access system in which microfilm jackets of the type disclosed in the above-identified Engelstein and Dorman patents are stacked in a file cartridge, a picker mechanism being provided which is adapted to select and extract any jacket from the cartridge regardless of its location therein. But because the jackets in the cartridge tend to stick together, in removing one jacket, the adjacent non-selected jackets may also be withdrawn. To overcome this drawback, the picker mechanism is vibrated in order to dislodge the unwanted jackets.
Commercially available transparent multi-channel jackets of the above-identified type are provided with an upper extension margin having a translucent coating on which one may print a title or other information relevant to the microfilm data contained in the jacket. It is now possible, using computer-controlled printers, to feed these jackets into a printer for titling, the jackets being taken one at a time from a stack. However, should an automatic feeder mechanism be used, the same difficulty is experienced in this automatic feeding action as in the random access system previously mentioned; for when the feeder mechanism removes a jacket from the top of the stack, the underlying jacket will at the same time be withdrawn should it stick to the top jacket.
In a computer-operated printer, the computer is programmed to instruct the printer to title the jackets successively fed thereto in a manner appropriate to each jacket. Should two or three jackets be fed at one time into the printer, this will upset the feed sequence and result in mistitling of the jackets. And since existing jackets tend to stick together, the only way one can properly feed such jackets into a printer for titling is by hand, one at a time.
The need exists, therefore, for a transparent multi-channel jacket that does not block, this being the term used in the trade to describe a situation in which the jackets in a stack tend to stick together and therefore block.
The present invention is concerned only with jackets whose panels are formed of polymeric material such as polyethylene terephthalate film (Mylar). While jackets of this material tend to stick together or block, this adhesion is not due to any tacky or sticky quality of the film. Mylar film is very smooth and devoid of any natural stickiness in the sense of it being viscous or gluey. The apparent stickiness is a consequence of the film's coefficient of static friction, and it is therefore necessary to review briefly the nature of friction so that this phenomenon and its relevance to the present invention are fully understood.
As applied to a solid body in rubbing contact with another, the term "friction" refers to the resistance to movement which is encountered when one seeks by way of an applied shear force to tangentially displace the solid body. On the atomic scale, even the most highly polished surface is far from perfectly planar. Thus when two finely polished metal plates are placed into intimate contact with each other, the actual microscopic area of contact is formed by a myriad of contact points and is much less than the apparent macroscopic area of contact.
Actual contact points tend to cold weld together, this bonding action taking place because at each minute contact point, the molecules on opposite sides thereof are so close together that they exert a strong intermolecular force on each other. When, therefore, one body is pulled across another, the frictional resistance therebetween can only be overcome by rupturing thousand of minute cold welds.
The coefficient of friction (u) is defined as the ratio of the force of friction (f.sub.r) to the normal force (N), the latter being the force pressing the rubbing surfaces together. Thus should the shear force necessary to overcome friction to effect tangential movement of the body be greater than the normal force, the coefficient of friction will then exceed 1 in value, but if it is less than the normal force, it will have a decimal value, such as 0.6 or 0.45.
Friction is invariably significantly greater at the start of motion than it is once motion has commenced. Hence one must distinguish between the coefficient of static friction (u.sub.s), which is a measure of the limiting friction which prevails just before motion starts, and the coefficient of kinetic friction (u.sub.k), which is a smaller coefficient that applies to the condition which prevails when uniform motion is in progress. In order to determine the coefficient of static friction, a block is placed on an inclined plane, and the plane is then gradually tilted until an angle is reached at which the block just slides down. The tangent of the angle of inclination is equal to the coefficient of friction between the block and the plane.
A number of processes occur at the interface of two solids which tend to inhibit sliding motion and therefore contribute to friction. In the history of technology, the question of which one of these processes is mainly responsible for friction has been the subject of extensive study. Until about 1940, it was thought that the main source of friction was surface roughness in that work had to be done during sliding motion to lift one surface over the high spots of the other surface in rubbing contact therewith.
More recently, the importance of surface roughness has been discounted, first because the work done in sliding up a high spot on the surface is in good measure recovered on the down slide. Indeed, experimental testing has demonstrated that for most sliding systems, the coefficients of friction are largely independent of surface roughness.
Thus a surface which is extremely smooth to the touch, such as cleaved mica, which is smooth to within one atomic diameter, nevertheless gives rise to friction which is as great or greater than that experienced with ordinary surfaces. By the same token, a Mylar film which has an extremely smooth surface has a high coefficient of friction.
The modern theory of friction attributes friction primarily to intermolecular bonds at the microscopic points of actual contact between two solids which are in rubbing relationship. Friction therefore mainly represents the force required to break these intermolecular bonds. For a more detailed analysis of the modern theory of friction, reference is made to the section on friction (page 405 et seq.) in the 1983 McGraw-Hill Encyclopedia of Engineering, edited by S. P. Parker.
In order to enhance the slip characteristics of polymeric film so that when the film is wound on a roll, the layers thereof have sufficient slip to permit proper coiling, it is known to coat the surface of the film with segregated nodules of silicon dioxide.
Thus in the 1974 patent to Anderson et al. U.S. Pat. No. 3,808,027, whose entire disclosure is incorporated herein by reference, the slip characteristics of the film are improved by first bringing the film surface in contact with water vapor to render it polar. The polar surface is then brought into contact with a hydrolyzing tetrafunctional silicon compound for a brief period of time sufficient to form discrete nodules of silicon dioxide which are bonded onto the film surface. These minute nodules bring about no appreciable change in the clarity or other optical properties of the transparent film.
A Mylar film treated in the manner disclosed in the Anderson et al. patent has improved slip to an extent which facilitates winding and other film handling operations. However, this treatment does not impart to the film a sufficiently reduced coefficient of static friction to prevent or minimize blocking when the treated film forms the panels of a transparent multi-channel jacket. Because jackets whose panels are made from such treated film tend to stick together, it becomes necessary, when the jackets are stacked, to employ special expedients to dislodge unwanted jackets when extracting a desired jacket from the stack.